This invention is related to ophthalmic lenses and, in particular embodiments, to novel progressive power and multifocal ophthalmic lens in which a surface of the lens has distance, near and intermediate viewing zones with boundaries and contours which provide desired optical properties over the surface of the lens. Disclosed methods permit the making of lenses with these properties while providing flexibility in the selection of wearer-adapted parameters.
The direct ancestor of the progressive multifocal lens is the bifocal lens which consists of a distance viewing zone of one optical power and a near viewing zone of another optical power, located adjacent the distance zone. Each zone may have a convex spherical surface with an abrupt change in curvature at a boundary where the zones abut. These lenses have the disdvantage that they are limited to two, more or less fixed optical powers. The change in power at the zone boundary can produce undesirable optical effects for the wearer. In addition, the zone boundaries are sometimes considered unattractive when the lens is viewed on the wearer.
Over the last 70 or 80 years attempts have been made to achieve a progressive increase in optical power from a distance viewing part of a lens to a near reading part of the lens. In so doing astigmatism and distortion are necessarily introduced into the lens.
British Pat. No. 775,007, to Jeffree, discusses at page 1 problems faced by the lens designer in achieving an acceptable lens of this type. One design technique mentioned by Jeffree is to maintain changes in horizontal and vertical curvature of the lens at the same magnitude. This is said to result in a lens with no unwanted astigmatism along a lens meridian, but such a lens is said to have increasingly severe oblique astigmatic errors are increasing lateral distances from the meridian. On the other hand, according to Jeffree, it is possible, by making the changes in horizontal curvature less than that of the vertical curvature to an appropriate degree, to produce lenses with no unwanted astigmatism of the type that increases with lateral distance from the meridian, but instead with unwanted astigmatism dependent on height from the bottom of the lens. A variety of compromises between these two cases is said to be possible.
Other designers have tried to minimize the effect of the astigmatism introduced into the lens. Some designers have attempted to concentrate the astigmatism into relatively unused portions of the lens. See, for example, British Pat. No. GB 2,056,106A to Younger Manufacturing Company; British Pat. No. GB 2,069,714A to Itek Corporation; U.S. Pat. No. 4,056,311 to Winthrop. Other designers have attempted to spread the astigmatism over a larger portion of the lens, thus decreasing the maximum value of astigmatism. See the comparative examples of U.S. Pat. No. 4,315,673 to Guilino et al.
Some prior art designs have as a goal the making of a progressive lens with substantially no astigmatism in a large distance viewing zone. An example of such a lens can be seen in FIG. 1 which is a reproduction of FIG. 4 of U.S. Pat. No. 3,785,724 to Cretin-Maitenaz. The resulting boundaries between the distance viewing zone and the progressive portion of the lens tend to be more apparent to the eye.
In other prior art lenses, zone boundaries are more diffuse and not so apparent on moving from the distance viewing zone to the progressive zone. An example of such a design is illustrated in FIG. 2, which is a reproduction of FIG. 7 of British Pat. No. 2,058,391A to Suwa Seikosha. The design methods of the present invention are adaptable to produce lenses with both sharp and diffuse zone boundaries. To the extent that the above-mentioned art is understandable, it is not believed to foreshadow the techniques of the present invention.
Certain characteristics in the progressive lens are desirable. The intermediate zone should join the near and distance zones in a cosmetically acceptable way, in the sense that no discontinuities in the lens should be visible to people observing the lens on the wearer. The intermediate zone should be optically acceptable in the sense that there should be a line or corridor, called the eye path, along which the line of vision moves while going between the distance and near zones and along which the optical power of the lens increases more or less uniformly. The quality of vision along the eye path should be as good as possible to permit some intermediate viewing through the intermediate zone and to provide a comfortable transition in powers for the eye between the near and distance zone. This result is furthered by minimizing or removing astigmatism along the eye path. It is possible to select an eye path with a slant to accommodate the natural convergence of the eyes. It may also be desirable to select particular shapes for the zones. Examples of zones of various shapes are illustrated in British Pat. No. 2,092,772A to Winthrop.
In order to understand problems associated with the provision of these desirable lens characteristics, it is helpful to understand how such lenses are made and dispensed. Finished progressive lenses are typically made from progressive lens blanks, i.e. optical bodies having a finished convex front progressive surface and a unfinished rear concave surface. The rear surface is normally finished to produce a lens which meets the wearer's prescription. However, to provide a range of distance and near zone optical powers which permit optimum fitting of the lens to most presbyopic patients, a family of progressive lens blanks with different distance and near zone powers should be available. In such a family, each blank will incorporate a similar selection and/or optimization of some or all of the above-mentioned basic design characteristics. Such a consistency is an aid in the designing, manufacturing, testing and prescribing of the lenses.
However, it is not a simple matter to choose an appropriate generalized expression to describe the progressive properties of a family of lens blanks. Such an expression needs to be one which readily permits alteration of distance zone power and near zone power, to obtain the desired specific characteristics of one of the family of lens blanks. In addition the design techniques, should, advantageously permit generation of different families of lenses, embodying different selections of other parameters such as zone size and placement, eye path location, etc.
Accordingly, it is an object of the present invention to provide a method for making lenses in which design parameters may be easily manipulated to produce progressive lenses having varying ophthalmic properties.
It is another object of the present invention to provide a method for making a progressive lens in which astigmatism and distortion are selectively distributed to improve optical and/or cosmetic properties of the lens.
It is another object of the present invention to provide a method for making a progressive lens in which the optical power, shape, size and/or location of the distance or near zones may be easily modified from lens to lens.
It is another object of the present invention to provide a method for making a progressive lens in which selected design parameters may be easily modified to improve astigmatism or distortion characteristics of the lens.
It is another object of the present invention to provide progressive lenses and progressive lens families with a large distance viewing zone, a large near viewing zone, a wide eye path corridor, and little or no astigmatism along the eye path.
These and other objects and features will be apparent from this written description and the appended drawings.